By Caroline Humer (Reuters) - Insurers and doctors are testing a way to pay for healthcare that has been more common in the corporate suite than the emergency room - paying for better performance, betting it is the key to controlling runaway costs. Both private insurance plans and Medicare plans in hundreds of locations around the country are using incentives to try to cut healthcare spending and still keep Americans healthy. After a few years of pilot programs and studies, companies as large as Intel Corp. are offering these plans to employees this year. ...